In an effort to boost HPV vaccination rates, the country's 69 National Cancer Institute-designated cancer centers on Wednesday endorsed recently revised federal guidelines that say only two doses of the vaccine are needed for younger adolescents.

The U.S. Centers for Disease Control and Prevention revised its guidance in October. Previously, it recommended three shots of the vaccine, which wards off infection with sexually transmitted strains of the virus that causes cervical cancer and some rarer head, neck and genital cancers. Although the vaccine was hailed as a breakthrough when it was approved in 2006, doctors and parents have been slow to embrace the immunization, put off by its novelty, link to sex, and the complexity of the three-shot regimen.

"The vaccination is our best defense in stopping HPV infection in our youth and preventing HPV-related cancers in the first place," said Richard I. Fisher, president and CEO of Fox Chase Cancer Center, one of the 69 centers. "There are plenty of cancers we can't prevent, but we have the tools to prevent HPV-related cancers."

The two-dose regimen is expected to improve vaccination rates. Athough the rates have been inching upward, they are far below those of other childhood immunizations. In 2015, about half of boys and 63 percent of girls ages 13 to 17 had gotten at least one HPV shot. Only 13 percent of boys and 37 percent of girls had gotten all three doses.

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